1. In-depth knowledge. As a sports writer, it's very important that you know and deeply understand the sports that you are writing about. That is why, it is highly recommended that you focus your attention on those sports that you're playing in the past or those that you find really, really interesting. It will also help if you watch the games and if you do your research. I am pretty sure that there are a lot of traditional books and magazines about any sport that you can think of.
2. High level writing skills. Having in-depth working knowledge on your chosen sports is not enough. As a writer, it's important that you know how to put together great content that your readers will find not only informative but entertaining as well. Improve your writing skills by simply reading as many sports articles as possible, by attending sports writing seminars, and by learning from renowned sports writers.
3. Topics. There are so many topics that you can cover when you are a sports writer. However, if you really want to capture the attention of your target audience, you must focus your attention on those that they find really, really interesting. Aside from writing about the latest games, you can also write human interest stories. For example, you can write problems that affect the lives of certain players (like Tiger Woods and his marital problem) or write an article about certain NBA teams who are doing charity work. These types of topics will surely help you get your audience to read your articles.